Chapter: Civil Surveying - Theodolite Surveying

Terminology of Theodolite

It is important to clearly understand the terms associated with the theodolite and its use and meaning. The following are some important terms and their definitions.

Terminology of Theodolite

It is important to clearly understand the terms associated with
the theodolite and its use and meaning. The following are some important terms
and their definitions.

Vertical axis It is a line passing
through the centre of the horizontal circle andperpendicular to it. The
vertical axis is perpendicular to the line of sight and the trunnion axis or
the horizontal axis. The instrument is rotated about this axis for sighting
different points.

Horizontal axis It is the
axis about which the telescope rotates when rotated ina vertical plane.
This axis is perpendicular to the line of collimation and the verti-cal axis.

Telescope axis It is the
line joining the optical centre of the object glass to thecentre of the
eyepiece.

Line of collimation It is the
line joining the intersection of the cross hairs to theoptical centre
of the object glass and its continuation. This is also called the line of
sight.

Axis of the bubble tube It is the
line tangential to the longitudinal curve of thebubble tube at its
centre.

Centring Centring the theodolite
means setting up the theodolite exactly over thestation mark. At this
position the plumb bob attached to the base of the instrument lies exactly over
the station mark.

Transiting It is the process of rotating
the telescope about the horizontal axisthrough 180 o . The telescope
points in the opposite direction after transiting. This process is also known
as plunging or reversing.

Swinging It is the process of
rotating the telescope about the vertical axis forthe purpose of
pointing the telescope in different directions. The right swing is a rotation
in the clockwise direction and the left swing is a rotation in the
counter-clockwise direction.

Face-left or normal position This
is the position in which as the sighting is done,the vertical circle is
to the left of the observer.

Face-right or inverted position This
is the position in which as the sighting isdone, the vertical circle is
to the right of the observer.

Changing face It is the operation of
changing from face left to face right and viceversa. This is done by
transiting the telescope and swinging it through 180 o .

Face-left observation It is the
reading taken when the instrument is in the normalor face-left
position.

Face-right observation It is the
reading taken when the instrument is in theinverted or face-right
position.